This invention relates generally to implantable devices such as heart pacemakers and, more particularly, to telemetry apparatus and methods for use in transmitting data from such implantable devices.
Implantable devices of this kind are becoming increasingly sophisticated and now are commonly configured to transmit, or telemeter, large amounts of data to an external device, commonly called a programmer. In the case of a heart pacemaker, for example, it is frequently desired to transmit data representing the real time electrocardiogram (ECG) signal from one or more chambers of the heart and, in addition, to transmit other data accumulated over time in an internal memory.
Data of this kind is preferably telemetered at a data rate of at least 8000 bits per second, which is most readily achievable if the pacemaker includes an antenna external to the pacemaker's case. Such a telemetry system is disclosed in U.S. Pat. No. 5,058,581, which is incorporated by reference. In the disclosed telemetry system, a carrier frequency in the range of 10 to 300 MHz is considered to be suitable.
As is well known, implanted devices of this kind use a low-energy battery and, consequently, any telemetry apparatus should consume very low energy. This low-energy requirement effectively precludes the use of a crystal-controlled oscillator and transmitter output stage for driving the antenna. This is because, after implantation, the connector/lead capacitance will increase by an uncontrolled amount, which detunes the transmitter output stage and reduces the transmitted power. Further, extraneous electromagnetic noise sources might sometimes be present at the transmitter frequency, which interferes with the telemetered signal and requires a higher transmitter power to be used.
It should, therefore, be appreciated that there is a need for an improved telemetry apparatus, and related method, for transmitting data from an implantable device such as a heart pacemaker with minimal use of the limited electrical energy available. The present invention fulfills this need.